It is important to detect a fuel injection condition, such as a fuel-injection-start timing, a fuel-injection-end timing, a fuel injection quantity and the like in order to accurately control an output torque and an emission of an internal combustion engine. Conventionally, it is known that an actual fuel injection condition is detected by sensing a fuel pressure in a fuel injection system, which is varied due to a fuel injection.
For example, JP-2008-144749A (US-2008-0228374A1) describes that an actual fuel-injection-start timing is detected by detecting a timing at which the fuel pressure in the fuel injection system starts to be decreased due to a start of the fuel injection and the fuel-injection-end timing is detected by detecting a timing at which the fuel pressure increase is stopped.
A fuel pressure sensor disposed in a common rail hardly detects a variation in the fuel pressure with high accuracy because the fuel pressure variation due to the fuel injection is attenuated in the common rail. JP-2008-144749A and JP-2000-265892A describe that a fuel pressure sensor is disposed in a fuel injector to detect the variation in the fuel pressure before the variation is attenuated in the common rail.
The present inventors has studied a method of computing the fuel-injection-end timing based on a pressure waveform detected by the pressure sensor disposed in a fuel injector, which method will be described hereinafter.
As shown in FIG. 13A, when a command signal for starting a fuel injection is outputted from an electronic control unit (ECU) at a fuel-injection-start command timing “Is”, a driving current supplied from an electronic driver unit (EDU) to a fuel injector starts to rise at the fuel-injection-start command timing “Is”. When a command signal for ending a fuel injection is outputted from the ECU at a fuel-injection-end command timing “Ie”, the driving current starts to fall at the fuel-injection-end command timing “Ie”. A detection pressure detected by the fuel pressure sensor varies as shown by a solid line “L1” in FIG. 13B.
It should be noted that the command signal for starting a fuel injection is referred to as a SFC-signal and the command signal for ending a fuel injection is referred to as an EFC-signal, hereinafter.
When the SFC-signal is outputted from the ECU at the fuel-injection-start command timing “Is” and an injection rate (injection quantity per unit time) increases, the detection pressure starts to decrease at a changing point “P3a” on the pressure waveform. Then, when the EFC-signal is outputted at the fuel-injection-end command timing “Ie” and the injection rate starts to decrease, the detection pressure starts to increase at a changing point “P7a” on the pressure waveform. Then, when the fuel injection ends and the injection rate becomes zero, the increase in the detection pressure ends at a changing point “P8a” on the pressure waveform.
A timing at which the changing point “P8a” appears is detected and the fuel-injection-end timing is computed based on its detection timing of the changing point “P8a”. Specifically, as shown by a solid line M1 in FIG. 13C, differential values are computed with respect to every detection pressure. After the SFC-signal is outputted and the differential value becomes maximum value, the differential value first becomes zero at a timing “t5”. This timing “t5” is detected as the timing at which the changing point “P8a” appears.
It should be noted that since the fuel in the fuel injector flows toward the injection ports by its inertia, the timing “t5” at which the changing point “P8a” appears is delayed by a specified time period T11 than an actual fuel-injection-end timing. In view of this point, the specified time period T11 is subtracted from the timing “t5” to compute a fuel-injection-end timing “R8”.
However, in a case that a multi-stage injection is performed, when an interval “IV” between n-th injection end and (n+1)th injection start is short, it may be occurred that a changing point “P3a” appears before the changing point “P8a” as shown by a dashed line L2 in FIG. 13B, wherein the changing point “P3a” represents a timing at which the detection pressure starts to decrease due to (n+1)th fuel injection start and the changing point “P8a” represents a timing at which an increase in the detection pressure ends due to n-th fuel injection end.
As a result, the differential values shift from a solid line M1 representing actual differential value to a dashed line M2, and the timing at which the differential value is zero shifts from the timing “t5” to the timing “tx”. Thus, a timing earlier than the actual fuel-injection-end timing “R8” may be erroneously detected as the fuel-injection-end timing.
Moreover, it is conceivable that noises overlapping on the pressure waveform may cause the deviation of the timing “t5”. Thus, even in a case that single-stage injection is performed or the interval “IV” is long, the above mentioned erroneous detection of the actual fuel-injection-end timing may be performed.